Electrodeposition coating is a method wherein aqueous paint is filled in a tank and paint components are electrophoretically moved and deposited onto a surface of an object to be coated present in the tank. The main application of this electrodeposition coating is anti-corrosion coating of car bodies and it is industrially important.
In general, an electrodeposition paint contains a pigment and a pigment grinding agent. In order to introduce a pigment into paint, a pigment paste obtained by grinding the pigment in an aqueous medium at a high concentration is used as an intermediate composition. To the pigment paste, a dispersing agent is formulated to give sufficient dispersion stability to the pigment paste. The dispersing agent is demanded to have the characteristic to increase dispersion stability of the pigment paste as well as performances to keep the paint stable for a prolonged period when the pigment paste is made into an electrodeposition paint by diluting it with an aqueous medium. In recent years, cationic resin is generally used for the pigment grinding agent of electrodeposition paint.
For instance, Japanese Kokai Publication Sho 63(1988)-23919 discloses a cationic resin which is obtained by addition of an alkylphenol and sulfide to an epoxy resin. However, since this resin has too strong hydrophilic property, the pigment paste using the resin has poor dispersion stability.
Japanese Kokai Publication Sho 53(1978)-47143 and Sho 56(1981)-2089 discloses a cationic resin which is prepared by reacting an epoxy resin with a half-blocked isocyanate and the reacting the obtained hydrophobic epoxy resin with an amine, phosphine or sulfide to give water solubility. The cationic resin described therein respectively contains ammonium salt groups, phosphonium salt groups or sulfonium salt groups.
However, since quaternary ammonium salt has a high base strength, it is impossible to obtain a coating having sufficient corrosion resistance even when the resin having the ammonium salt is used. Besides, the resin having the tertiary sulfonium salt has a low reactivity with sulfide and water solubility is insufficient and therefore, the electrodeposition paint using the resin has poor stability. The resin having phosphonium salt neither provides an electrodeposition paint of good stability because of insufficient hydrophillic property of phosphonium.